This invention relates to a polyvinyl acetate-modified phenolic resin composition having improved adhesion.
Polyvinyl acetate-modified phenolic resin is a thermosetting resin obtained by admixing an alcohol-soluble phenolic resin of the resol type with a polyvinyl acetate resin, and is characterized by properties different from those of alcohol-soluble resol type phenolic resin and of the vinyl acetate resin. Alcohol soluble resol resins (typified by resol resins soluble in lower monohydric alcohols such as methanol) are more particularly described in D. F. Gould "Phenolic Resins" Reinhold Publishing Corporation 1959, Chapter 3, the pertinent disclosure of which is incorporated herein by reference. Mixtures of phenolic resins and polyvinyl acetate are disclosed by Gould op cit page 95, lines 26-29. More specifically the polyvinyl-acetate modified phenolic resin is an advantageous adhesive in that it can meet the higher adhesive durability standard required in the specification of Type I plywood of JAS (Japanese Agricultural Standards) which is difficult to achieve with vinyl acetate resins. It is also advantageous in that the applicators and related equipment used with the resin mixture are washable with water, which advantage is not realizable in the case of alcohol-soluble resol type phenolic resins.
Adhesive compositions are prepared from such resins by the addition, directly before use, of a strongly acidic hardener such as benzenesulfonic acid or para-toluenesulfonic acid (used in sufficient proportion in the adhesive composition to impart a pH of about 0.1 to about 1.0 to the adhesive composition) which is applied onto the adherend. The resin-containing adherends are pressed under a pressure of about 2-15 kg/cm.sup.2 for about 5-20 hours at 15.degree.-40.degree. C. to cure the resin, thereby completing the adhesion. However, in comparison with alcohol-soluble resol type phenolic resins or vinyl acetate resins, the polyvinyl acetate-modified phenolic resins generally have a serious drawback in showing a significant loss in bonding strength when the socalled open assembly time, or the period between the coating of adhesive composition on the adherend and curing of the resin is long i.e. about 10 minutes or longer. The necessity of limiting open assembly time in using conventional polyvinyl acetate-modified phenolic resin compositions inevitably necessitates rapid processing in wood working plants, and eventually leads to defective adhesion of the resin product, particularly at high ambient temperatures.
This phenomenon can be ascribed to an insufficient flow of the adhesive composition when compressed after a prolonged open assembly time, and an insufficient bonding strength resulting therefrom due to the faster hardening speed of such an adhesive composition in comparison with that of ordinary alcohol-soluble resol type phenolic resins or of vinyl acetate resins, as the adhesive composition of polyvinyl acetate-modified phenolic resin containing a hardener already starts hardening on the adherend at normally encountered temperatures above 15.degree. C., and as the vinyl acetate resin used as its modifier is originally provided with a relatively fast drying speed.